Image density control apparatus

ABSTRACT

An apparatus in which the potential of a sample electrostatic latent image recorded on a photoconductive surface is detected for controlling the density of toner particles deposited on a single color electrostatic latent image recorded thereon.

United States Patent [191 McVeigh et al.

[451 June 11, 1974 IMAGE DENSITY CONTROL APPARATUS [75] Inventors: James H. McVeigh; George N.

Tsilibes, both of Rochester, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: May 17, 1973 [2]] Appl. No.: 361,112

[52] U.S. Cl 355/3 R, 355/4, 96/12, 117/17.5, 118/637 [51] Int. Cl G03g 15/00 [58] Field of Search 355/3, 4; 118/637;

[56] References Cited UNITED STATES PATENTS 2,956,487 10/1960 Giaimo 118/7 UX 3,674,532 7/1972 Morse 355/3 X Primary Examiner-Robert P. Greiner Attorney, Agent, or FirmH. Fleischer; James J. Ralabate; C. A. Green [57] ABSTRACT An apparatus in which the potential of a sample electrostatic latent image recorded on a photoconductive surface is detected for controlling the density of toner particles deposited on a single color electrostatic latent image recorded thereon.

10 Claims, 4 Drawing Figures Pmmmmnm 3.815988 v sum sur- 3 p ER REGULATING SU LY CIRCUITRY a 1 IMAGE DENSITY CONTROL APPARATUS BACKGROUND OF THE INVENTION This invention relates generally to a multi-color electrophotographic printing machine, and more particularly concerns an apparatus for controlling the density of toner particles deposited on a single color electrostatic latent image recorded on a charged photoconductive surface.

In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of an original document. Exposure of the photoconductive surface records thereon an electrostatic latent image corresponding to the original document. The electrostatic latent image is then rendered visible by depositing thereon toner particles which adhere electrostatically, in image configuration, thereto. Thereafter, the toner powder image may be transferred to a sheet of support material. The toner powder image is, then, permanently affixed to the support material to provide a copy of the original document. The foregoing process was originally disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.

Multi-color electrophotographic printing is similar to the heretofore discussed process with the following exceptions. Rather than forming a total light image of the original document, the light image is filtered producing a single color light image which is a partial light image of the original. The foregoing single color light image exposes the charged photoconductive surface to record thereon a single color electrostatic latent image. This single color electrostatic latent image is developed with toner particles of a color complementary to the single color light image. Subsequently, the single color toner powder image is transferred to the sheet of support material. The foregoing process is repeated a plurality of cycles with differently colored light images and the respective complementary colored toner particles. Each single color toner powder image is transferred to the support material superimposed in registration with the prior toner powder image to form a composite multilayer powder image thereon. This multi-layered toner powder image is coalesced and permanently affixed to the support material forming a composite image corresponding in color to the original document.

It is apparent that in multi-color electrophotographic printing machines, the characteristics of the photoconductive surface are critical. Preferably, the electrical characteristics of the photoconductive surface should remain substantially constant. However, it has been found that the electrical characteristics of the photoconductive surfaces will vary with temperature changes or with continuous usage thereof. Hence, it is extremely difficult to maintain substantially the same potential on the photoconductive surface for light images projected thereon having substantially identical intensities. Moreover, electrophotographic printing machines frequently utilize magnetic brushes to produce viewable toner'powder images on the electrostatic latent image recorded on the photoconductive surface. Toner particles are attracted from the magnetic brush to the charged photoconductive surface.

In multi-color electrophotographic printing, the imaged areas are developed with the toner particles whereas the non-image areas remain substantially devoid of toner particles. However, it is evident that some SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided an apparatus for controlling the density of toner particles deposited on a single color electrostatic latent image recorded on a charged photoconductive surface.

In the present instance, the apparatus includes at least one neutral density sample, illuminating means, toner particle depositing means, and sensing and electrical biasing means. Preferably, the neutral density sample has a pre-sele cted density corresponding to substantially about a predetermined cut-off density for the single color electrostatic latent image recorded on the photoconductive surface. The illuminating means irradiates the neutral density sample and projects the light image formed thereof onto the charged photoconductive surface. In this way, a sample electrostatic latent image is recorded on the photoconductive surface. The sample electrostatic latent image has a potential intermediate that of the single color electrostatic latent image and the non-image regions of the charged photoconductive surface. The potential of the sample electrostatic latent image recorded on the charged photoconductive surface is detected by the sensing and electrical biasing means. Pursuant to the present invention, the sensing and electrical biasing means electrically bias the toner particle depositing means to a potential corresponding to that of the sample electrostatic latent image recorded on the charged photoconductive surface. Hence, toner particles are deposited on regions of the photoconductive surface having a potential greater than'the potential of the sample electrostatic latent image.

BRIEF DESCRIPTION OF THE DRAWINGS potential of the sample electrostatic latent image recorded on the photoconductive surface; and.

FIG. 4 is a schematic circuit diagram for periodically sampling the sensed sample electrostatic latent image potential.

While the present invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings, FIG. 1 schematically illustrates a multi-color electrophotographic printing machine employing the present invention. In the drawings, like reference numerals have been used throughout to designate like elements. The multi-color electrophotographic printing machine shown schematically in FIG. 1, illustrates the various components used to produce multi-color copies from a colored original. Although the apparatus of the present invention is particularly well adapted for use in an electrophotographic printing machine, it will become evident from the following description that it is equally well suited for use in a wide variety of electrophotographic printing machines, and is not necessarily limited to the particular embodiment shown therein.

As shown in FIG. 1, the printing machine employs a drum 10 having a photoconductive surface 12 secured thereto and entrained about the exterior circumferential surface thereof. Drum 10 is mounted rotatably within the machine frame (not shown). A series of processing stations are positioned such that as drum I rotates in the direction of arrow 14, photoconductive surface 12 passes sequentially therethrough. Drum is driven at a predetermined speed relative to the other machine operating mechanisms by a common drive motor (not shown). One type of suitable photoconductive material is disclosed in US. Pat. No. 3,655,377, issued to Sechak in 1972. A timing wheel is mounted in the region of one end of drum 10 and adapted to trigger the logic circuitry of the printing machine. This coordinates the various machine operations with one another to produce the proper sequence of the events at the appropriate processing stations.

Initially, drum 10 moves photoconductive surface 12 through charging station A. Charging station A has positioned thereat a corona generating device, indicated generally at 1 6. Corona generating device 16 extends in a generally longitudinal direction transversely across photoconductive surface 12. This readily enables corona generating device 16 to charge photoconductive surface 12 to a relatively high substantially uniform potential. Preferably, corona generating device 16 is of the type described in US. Pat. No. 2,778,946 issued to Mayo in 1957.

Drum 10, thereafter, is rotated to exposure station B. Exposure station B includes thereat a moving lens system, generally designated by the reference number 18, and a color filter mechanism shown generally at 20. A suitable moving lens system is disclosed in U.S. Pat. No. 3,062,108 issued to Mayo in 1962, and a suitable color filter mechanism is described in co-pending application Ser. No. 830,282 filed in 1969. Disc 22 has a plurality of neutral density samples (in this case 3) disposed thereon. Disc 22 is mounted rotatably in the printing machine and is disposed beneath transparent platen 24 within the half angle of the optical system. Before the light source lamps indicated generally by the reference numeral 26, begin to scan, they will be actuated to illuminate one of the neutral density samples. In this way,

a sample electrostatic latent image is recorded, on photoconductive surface 12 as drum 10 rotates. Lamps 26 are stationary and the appropriate filter is positioned in filter 20 forming a sample electrostatic latent image on photoconductive surface I2 which is a strip discharged to the desired potential. The potential of the sample electrostatic latent image recorded on photoconductive surface 12 is detected by probe 28, i.e. a suitable electrometer disposed adjacent to photoconductive surface 12 intermediate exposure station 8 and development station C. The electrical output signal from probe 28 is processed by circuit elements 30 which regulate voltage source or power supply 84 adjusting the bias voltage of the respective developer unit having toner particles complementary in color to the filter of filter mechanism 20. Preferably, disc 22 includes three equally spaced neutral density samples located about the periphery thereof. Sample 32 is a neutral density sample for green separation, sample 34 is a neutral density sample for red separation and sample 36 is a neutral sample for blue separation. Preferably the green separation sample has a density of 0.32, the blue separation sample a density of 0.35, and the red separation sample a density of 0.15. The appropriate neutral density sample is illuminated by light source 26 to produce a sample electrostatic image corresponding to a predetermined development density for the filter being used, i.e. a green filter will have neutral density sample 32 illuminated forming a sample electrostatic latent image corresponding to the predetermined development density for the green separation.

In multi-color electrophotographic printing, a single color light image exposes the charged photoconductive surface. The potential on the charged photoconductive surface in the area irradiated by the single color light image is reduced. The potential of the charged photoconductive surface in the non-irradiated areas remain substantially unchanged. During development, toner particles, complementary in color to the single color light image, are deposited on the photoconductive surface. The irradiated areas remain substantially devoid of toner particles. The development system is biased such that the potential thereof is intermediate the irradiated and non-irradiated areas. In this way, toner particles are attracted to the non-irradiated areas from the development system since the potential of the nonirradiated areas is greater than the potential of the development system, whereas toner particles are not attracted to the irradiated areas inasmuch as the charge thereof is less thin than that of the development system. Each of the neutral density samples form a sample electrostatic latent image. The charge of the sample electrostatic latent image is greater than that of the irradiated areas and less than that of the non-irradiated areas. The developer unit is adjusted to the potential of the sample electrostatic latent image. Thus, toner particles are attracted to all regions of the charged photoconductive surface having a potential greater than that of the sample electrostatic latent image. The potential of the sample electrostatic latent image corresponds to the washout density of the single color toner powder image, i.e. the potential beneath which development of the single color electrostatic latent image does not occur. However, if the potential of the single color electrostatic latent image is greater than that of the sample electrostatic latent image development will occur.

With continued reference to the FIG. 1, after the sample electrostatic latent image is formed on the charged photoconductive surface, an original document 25, such as a book, sheet of paper, or the like, disposed upon transparent viewing platen 24 is scanned. Lamps 26 and lens 18 move in a timed relation with drum to scan successive incremental areas of original document 25 disposed upon platen 24. This creates a flowing light image of original document 25 which is projected onto charged photoconductive surface 12. Filter mechanism 20 is adapted to interpose selected color filters into the optical light path. The appropriate color filter operates on the light rays passing through lens 18 to record an electrostatic latent image on photoconductive surface 12 corresponding to a preselected spectral region of the electromagnetic wave spectrum, heretofore referred to as a single color electrostatic latent image.

After exposure, drum 10 rotates the single color electrostatic latent image recorded on photoconductive surface 12 to development station C. Development station C includes thereat three individual developer units, generally indicated by the reference numerals 38, 40, and 42. A suitable development system employing a plurality of developer units is disclosed in co-pending application Ser. No. 255,259, filed in 1970. Preferably, the developer units are all of a type generally referred to as magnetic brush developer units. A typical magnetic brush developer unit utilizes a magnetizable developer mix having carrier granules and toner particles. The developer mix is continually brought through a directional flux field to form a brush thereof. Each developer unit includes a developer roll 86, 88 and 90 (FIG. 3) electrically biased to the appropriate potential such that toner particles are attracted to the image areas (non-irradiated areas) rather than the non-image areas (irradiated areas) of the photoconductive surface 12. The potential applied to the developer roll is substantially equal to that of the sample electrostatic latent image recorded on photoconductive surface 12 and detected by probe 28. The single'color electrostatic latent image recorded on photoconductive surface 12 is developed by bringing the brush of developer mix into contact therewith. Each of the respective developer units contain discretely colored toner particles corresponding to the complement of the spectral region of the wavelength of light transmitted through filter 21, e.g. a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles therein, blue and red latent images are developed with yellow and cyan toner particles, respectively.

Drum 10 is, next, rotated to transfer station D where the toner powder image adhering electrostatically to photoconductive surface 12 is transferred to a sheet of final support material 44. Support material 44 may be plain paper, or a sheet of transparent, thermoplastic material. A transfer roll, shown generally at 46, rotates support material 44 in the direction of arrow 48. Transfer roll 46 is electrically biased to a potential of sufficient magnitude and polarity to electrostatically attract toner particles from photoconductive surface 12 to support material 44. U.S. Pat. No. 3,612,677, issued to Langdon et al. in 1972, discloses a suitable electrically biased transfer roll. Transfer roll 46 is arranged to rotate in synchronism with drum 10, Le. transfer roll 46 and drum l0 rotate at substantially the same angular velocity and have substantially the same outer diameter. Inasmuch as support material 44 is secured to transfer roll 46 for movement therewith in a recirculating path, successive toner powder images may be transferred from photoconductive surface 12 to support material 44, in superimposed registration with one another. Hence, a multi-color toner powder image corresponding in color to the original document is formed on support material 44.

With continued reference to FIG. 1, the sheet feeding path for advancing support material 44 to transfer roll 46 will be briefly described hereinafter. A stack 50 of support material 44 is supported on tray 52. Feed roll 54, operatively associated with retard roll 56, separates and advances the uppermost sheet from stack 50. The advancing sheet moves into chute 58 and is directed into the nip of register rolls 60. Next, gripper fingers 62, mounted on transfer roll 46, releasably secure thereto support material 44 for movement therewith in a recirculating path.

After all of the discretely colored toner powder images have been transferred to support material 44, gripper fingers 62 space support material 44 from transfer roll 46. This enables stripper bar 64 to be interposed between support material 44 and transfer roll 46 separating support material 44 therefrom. After support material 44 is stripped from transfer roll 46, it is moved on endless belt conveyor 66 to fixing station E.

At station E, a suitable fuser, indicated generally at 68, coalesces and permanently affixes the toner powder image to support material 44. A typical fuser is described in U.S. Pat. No. 3,498,592 issued to Moser et al. in 1970. After the multi-layered toner powder image is fixed to support material 44, endless belt conveyors 68 and 70 advance support material 44 to catch tray 72. Catch tray 72 is readily accessible so that an operator may remove the final multi-color copy from the printing machine.

lnvariably, residual toner particles remain on photoconductive surface 12 after the transfer of the toner powder image therefrom to support material 44. These residual toner particles are removed from photoconductive surface 12 as it passes through cleaning station F. At cleaning station'F, residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on the residual toner particles and photoconductive surface. The neutralized toner particles are then removed from photoconductive surface 12 by rotatably mounted brush 76. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971. Brush 76 is positioned at cleaning station F and maintained in contact with photoconductive surface 12. Thus, the residual toner particles remaining on photoconductive surface 12, after each successive transfer operation, are readily removed therefrom.

Turning now to FIG. 2, there is shown lamp carriage 78 supporting a pair of light sources or lamps 26 thereon. Lamp carriage 78 is arranged to traverse platen 24 illuminating incremental widths of original document 25 disposed therein. A suitable belt drive system advances lamp carriage 78 in the direction of arrow 80 to scan successive incremental areas of the original document 25 and returns lamp carriage 78 in the direction of arrow 82 to the initial position. Disc 22 is mounted rotatably on the printing machine frame and is interposed between lamp carriage 78 and platen 24. Thus, when lamp carriage 78 is in the initial position, prior to the initiation of the scan cycle, disc 22 is indexed so that light source 26 illuminates one of the neutral density samples disposed thereon. For example,

in FIG. 1, neutral density sample 32 is shown in position to be illuminated. Light source 26 remains stationary as drum 10 rotates so that a sample electrostatic Iatent image corresponding in density to the neutral density sample is recorded on photoconductive surface 12.

Referring now to FIG. 3, there is shown developer units 38, 40 and 42, probe 28 and drum 10. Probe 28 is secured in the machine frame and positioned between exposure station B and development station C. Probe 28 is seated within the machines support housing and arranged to detect the sample electrostatic latent image recorded on photoconductive surface 12. A light image of the neutral density sample is projected onto the charged photoconductive surface recording a sample electrostatic latent image thereon. The sample electrostatic latent image is detected by probe 28. The machine logic is arranged to generate a signal during each print cycle initiating the formation of the sample electrostatic latent image. In practice, the signal is generated when light source 26 is in the initial position prior to scanning of original document 25. A voltage indicative of the sample electrostatic latent image is sensed by probe 28 and processed by electrical circuitry 30 to produce an electrical output signal regulating voltage source or variable power supply 84. Power supply 84 is operatively connected to developer rolls 86, 88 and 90, respectively, of the corresponding developer units 38, 40, and 42. Power supply 84 regulates the electrical potential applied to the respective developer rolls 86, 88 and 90. In this way, each of the developer rolls is selectively biased to a potential substantially identical to that of the appropriate sample electrostatic latent image potential recorded on photoconductive surface 12. Thus, the developer roll potential is intermediate photoconductive the potential of the irradiated and non-irradiated areas in photoconductive surface 12. The signal generated by the machine logic has a pulse of sufficient duration to de-energize the drive of lamp carriage 78 when light source 26 is in the initial position. This enables disc 22 to index such that a neutral density sample is illuminated by light source 26. The resulting light image thereof is projected onto the moving photoconductive surface forming the sample electrostatic latent image thereon. After the sample electrostatic latent image is formed, a second pulse of sufficient duration is generated by the machine logic actuating the drive system of lamp carriage 78 so that light source 26 illuminates incremental portions of original document 25 as it moves thereacross. This creates a single color electrostatic latent image on photoconductive surface 12 after the corresponding sample electrostatic latent image is recorded thereon. As drum 10 rotates the sample electrostatic latent image recorded thereon, it passes adjacent to probe 28. Probe 28 senses the potential of the sample electrostatic latent image and develops a voltage signal indicative thereof.

As shown in FIG. 4, the voltage signal from probe 28 is processed by unity gain amplifier 92. A suitable amplifier having a high impedance can be utilized in conjunction with the probe of the present invention. The electrical output from amplifier 92, is transmitted through two successive amplifier stages 94 and 96, and then applied to a hold circuit including a high impedance unity gain amplifier 98 and a capacitor 100.

. However, the signal is initially prevented from passing to the hold circuit by normally open contact 102.

Referring once again to FIG. 4, probe 28 includes a sensing element 104 surrounded by an insulator 106. Insulator 106 is preferably fabricated from a material which is electrically insensitive to humidity changes and functions to maintain a high probe-to-ground resistance. Conductive shield 108 is disposed around insulator 106 and the output from amplifier 92 is fed back to shield 108. This maintains shield 108 at the same potential as amplifier 92 reducing current leakage from sensing element 104 to the surrounding electrical ground. The machine logic, preferably, includes suitable circuitry adapted to close contact at the appropriate time. Thus, the sample voltage is applied across the high impedance unity gain amplifier 98. Closing contact 102 causes two discrete conditions to occur. Initially, the sensed sample electrostatic latent image potential is applied across the high impedance amplifier 98 and secondarily, capacitor 100, in the hold circuitry, is charged to the sample electrostatic latent image potential. Termination of the signal from the machine logic after thesample electrostatic latent image has passed probe 28 permits contact v102 to return to its normally open position. However, the sample electrostatic latent image potential is stored on capacitor 100 and continues to be impressed across amplifier 98. Because of the high impedance of amplifier 98, a relatively constant output is maintained during the hold periods until the subsequent reclosing of contacts 102 provides a new sample electrostatic latent image potential. This output voltage is applied to power supply 84 (FIG. 3) holding the output voltage therefrom substantially constant until the next sample signal is received thereby. If the potential level of the next sample electrostatic latent image differs from that of the first sample electrostatic latent image, capacitor 100 is allowed to recharge to the new potential through contact 102 and through the circuitry of amplifier 96. The new sample electrostatic latent image potential is impressed across the high-impedance hold amplifier 98 and capacitor 100 is recharged to this new voltage. The output voltage is supplied to power supply high-voltage operational amplifier which holds the voltage output from power supply 84 substantially constant until the next signal is received. At the end of the sample period, contact 102 is again open and the hold circuit waits for the next sample. It is evident, therefore, that this type of arrangement permits the present apparatus to detect both increases and decreases in the potential of the sample electrostatic latent image recorded on photoconductive surface 12 while, substantially simultaneously therewith, generating a continuous control signal for regulating the potential applied to developer rolls 86, 88 and 90 of developer units 38, 40 and 42, respectively.

While the present invention has been described in connection with a single set of three neutral density samples, one skilled in the art will appreciate that the invention is not necessarily so limited and that a plurality of such sets may be utilized, each set corresponding to a prescribed set of conditions and having specified densities to achieve desired copy characteristics. Furthermore, while the present invention has been described as utilizing a disc, it will be apparent to one skilled in the art that the neutral density samples may be mounted on any suitable support arranged to be appropriately indexed, e.g. an endless conveyor belt.

ln recapitulation, it is apparent that the apparatus of the present invention controls the cut-off density of toner particles deposited on a single color electrostatic latent image recorded on a charged photoconductive surface. This is achieved by exposing the charged photoconductive surface to a neutral density sample having a pre-selected density corresponding to substantially about the predetermined cut-off density of the single color electrostatic latent image. In this way, a sample electrostatic latent image is recorded on the photoconductive surface. The potential of the sample electrostatic latent image is employed to electrically bias the developer roll of the corresponding magnetic brush developer unit to substantially the same potential. Thus, toner particles are attracted to those regions of the photoconductive surface having a potential greater than that of the sample electrostatic latent image. Inasmuch as the potential of the non-image region is substantially less than that of the image region, toner particles are not attracted thereto and the image region of photoconductive surface 12 has toner particles deposited thereon.

It is, therefore, evident that there has been provided, in accordance with the present invention, an apparatus for controlling the cut-off density of toner particles deposited on a single color electrostatic latent image recorded on a photoconductive surface that fully satisfies the objects, aims, and advantages set forth above. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.

What is claimed is:

1. An apparatus for controlling the cut-off density of toner particles deposited on a single color electrostatic latent image recorded on a charged photoconductive surface, including:

at least one neutral density sample having a preselected density corresponding to substantially about the predetermined cut-off density of the single color electrostatic latent image;

means for illuminating said neutral density sample and projecting the light image formed thereof onto the charged photoconductive surface to record thereon a sample electrostatic latent image having a potential intermediate the single color electrostatic latent image and the non-image regions of the charged photoconductive surface;

means for depositing toner particles, complementary in color to the single color electrostatic latent image, on the charged photoconductive surface; and

means for sensing the potential of the sample electrostatic latent image recorded on the charged photoconductive surface and electrically biasing said toner particle depositing means to a potential corresponding to the sample electrostatic latent image potential so that toner particles are deposited on regions of the photoconductive surface having a potential substantially greater than the potential of the sample electrostatic latent image.

2. An apparatus as recited in claim 1, wherein said sensing and biasing means includes;

a probe positioned adjacent to the photoconductive surface and arranged to detect the potential of the sample electrostatic latent image recorded thereon prior to toner particles being deposited on the photoconductive surface;

a voltage source for electrically biasing said toner particle depositing means; and

means, in electrical communication with said probe and said voltage source, for generating an electrical output signal indicative of the sample electrostatic latent image potential detected by said probe to regulate the output voltage of said voltage source.

3. An apparatus as recited in claim 2, wherein successive distinguishable single color electrostatic latent images are recorded on the charged photoconductive surface, including an indexable support member having a plurality of discrete neutral density samples disposed thereon, said support member being mounted in a lightreceiving relationship with said illuminating means, each of said neutral density samples having a preselected density substantially about the cut-off density of the respective single color electrostatic latent image corresponding thereto.

4. An apparatus as'recited in claim 3, wherein the neutral density samples disposed on said support member include:

a first neutral density sample for a green electrostatic latent image;

a second neutral density sample spaced from said first neutral density sample for a blue electrostatic latent image; and

a third neutral density sample spaced from said first and second neutral density samples for a red electrostatic latent image.

5. An apparatus as recited in claim 2, wherein said electrical signal generating means includes:

means for periodically sampling the sample electrostatic latent image potential detected by said probe; and

circuit means for analyzing the periodically detected sample electrostatic latent image potential and forming a continuous electrical output signal indicative thereof,

6. An electrophotographic printing machine of the type having a photoconductive surface, including:

means for charging the photoconductive surface to a substantially uniform potential;

at least one neutral density sample having a preselected density corresponding to substantially about the predetermined cut-off density of the single color electrostatic latent image;

means for exposing the charged photoconductive surface to a single color light image of an original document to record thereon a single color electrostatic latent image, said exposing means being arranged to illuminate said neutral density sample and project a light image thereof onto the charged photoconductive surface to record thereon a sample electrostatic latent image having a potential intermediate the single color electrostatic latent image and the non-image regions of the charged photoconductive surface;

means for depositing toner particles, complementary in color to the single color electrostatic latent image, on the charged photoconductive surface; and

means for sensing the potential of the sample electrostatic latent image recorded on the charged photoconductive surface and electrically biasing said toner particle depositing means to a potential corresponding to the sample electrostatic latent image potential so that toner particles are deposited on regions of the photoconductive surface having a potential substantially greater than the potential of the sample electrostatic latent image.

7. A printing machine as recited in claim 6, wherein said sensing and biasing means includes:

a probe positioned adjacent to the photoconductive surface and arranged to detect the potential of the sample electrostatic latent image recorded thereon prior to toner particles being deposited on the photoconductive surface;

a voltage source for electrically biasing said toner particle depositing means; and

means, in electrical communication with said probe and said voltage source, for generating an electrical output signal indicative of the sample electrostatic latent image potential detected by said probe to regulate the output voltage of said voltage source.

8. A printing machine as recited in claim 7, wherein successive distinguishable single color electrostatic latent images are recorded on the charged photoconductive surface, including an indexable support member having a plurality of discrete neutral density samples disposed thereon, said support member being mounted on the printing machine in a light-receiving relationship with said exposing means, each of said neutral density samples having a pre-selected density substantially about the cut-off density of the respective single color electrostatic latent image corresponding thereto.

9. A printing machine as recited in claim 8, wherein the neutral density samples disposed on said support member include:

a first neutral density sample for a green electrostatic latent image;

a second neutral density sample spaced from said first neutral density sample for a blue electrostatic latent image;

a third neutral density sample spaced from said first and second neutral density samples for a red electrostatie latent image.

10. A printing machine as recited in claim 7, wherein said electrical signal generating means includes means for periodically sampling the sample electrostatic image potential detected by said probe; and

dicative thereof. 

1. An apparatus for controlling the cut-off density of toner particles deposited on a single color electrostatic latent image recorded on a charged photoconductive surface, including: at least one neutral density sample having a pre-selected density corresponding to substantially about the predetermined cut-off density of the single color electrostatic latent image; means for illuminating said neutral density sample and projecting the light image formed thereof onto the charged photoconductive surface to record thereon a sample electrostatic latent image having a potential intermediate the single color electrostatic latent image and the non-image regions of the charged photoconductive surface; means for depositing toner particles, complementary in color to the single color electrostatic latent image, on the charged photoconductive surface; and means for sensing the potential of the sample electrostatic latent image recorded on the charged photoconductive surface and electrically biasing said toner particle depositing means to a potential corresponding to the sample electrostatic lAtent image potential so that toner particles are deposited on regions of the photoconductive surface having a potential substantially greater than the potential of the sample electrostatic latent image.
 2. An apparatus as recited in claim 1, wherein said sensing and biasing means includes; a probe positioned adjacent to the photoconductive surface and arranged to detect the potential of the sample electrostatic latent image recorded thereon prior to toner particles being deposited on the photoconductive surface; a voltage source for electrically biasing said toner particle depositing means; and means, in electrical communication with said probe and said voltage source, for generating an electrical output signal indicative of the sample electrostatic latent image potential detected by said probe to regulate the output voltage of said voltage source.
 3. An apparatus as recited in claim 2, wherein successive distinguishable single color electrostatic latent images are recorded on the charged photoconductive surface, including an indexable support member having a plurality of discrete neutral density samples disposed thereon, said support member being mounted in a light-receiving relationship with said illuminating means, each of said neutral density samples having a pre-selected density substantially about the cut-off density of the respective single color electrostatic latent image corresponding thereto.
 4. An apparatus as recited in claim 3, wherein the neutral density samples disposed on said support member include: a first neutral density sample for a green electrostatic latent image; a second neutral density sample spaced from said first neutral density sample for a blue electrostatic latent image; and a third neutral density sample spaced from said first and second neutral density samples for a red electrostatic latent image.
 5. An apparatus as recited in claim 2, wherein said electrical signal generating means includes: means for periodically sampling the sample electrostatic latent image potential detected by said probe; and circuit means for analyzing the periodically detected sample electrostatic latent image potential and forming a continuous electrical output signal indicative thereof.
 6. An electrophotographic printing machine of the type having a photoconductive surface, including: means for charging the photoconductive surface to a substantially uniform potential; at least one neutral density sample having a preselected density corresponding to substantially about the predetermined cut-off density of the single color electrostatic latent image; means for exposing the charged photoconductive surface to a single color light image of an original document to record thereon a single color electrostatic latent image, said exposing means being arranged to illuminate said neutral density sample and project a light image thereof onto the charged photoconductive surface to record thereon a sample electrostatic latent image having a potential intermediate the single color electrostatic latent image and the non-image regions of the charged photoconductive surface; means for depositing toner particles, complementary in color to the single color electrostatic latent image, on the charged photoconductive surface; and means for sensing the potential of the sample electrostatic latent image recorded on the charged photoconductive surface and electrically biasing said toner particle depositing means to a potential corresponding to the sample electrostatic latent image potential so that toner particles are deposited on regions of the photoconductive surface having a potential substantially greater than the potential of the sample electrostatic latent image.
 7. A printing machine as recited in claim 6, wherein said sensing and biasing means includes: a probe positioned adjacent to the photoconductive surface and arranged to detect the potential of the sample electrostatic latent image recOrded thereon prior to toner particles being deposited on the photoconductive surface; a voltage source for electrically biasing said toner particle depositing means; and means, in electrical communication with said probe and said voltage source, for generating an electrical output signal indicative of the sample electrostatic latent image potential detected by said probe to regulate the output voltage of said voltage source.
 8. A printing machine as recited in claim 7, wherein successive distinguishable single color electrostatic latent images are recorded on the charged photoconductive surface, including an indexable support member having a plurality of discrete neutral density samples disposed thereon, said support member being mounted on the printing machine in a light-receiving relationship with said exposing means, each of said neutral density samples having a pre-selected density substantially about the cut-off density of the respective single color electrostatic latent image corresponding thereto.
 9. A printing machine as recited in claim 8, wherein the neutral density samples disposed on said support member include: a first neutral density sample for a green electrostatic latent image; a second neutral density sample spaced from said first neutral density sample for a blue electrostatic latent image; a third neutral density sample spaced from said first and second neutral density samples for a red electrostatic latent image.
 10. A printing machine as recited in claim 7, wherein said electrical signal generating means includes means for periodically sampling the sample electrostatic image potential detected by said probe; and circuit means for analyzing the periodically detected sample electrostatic latent image potential and forming a continuous electrically output signal indicative thereof. 